The semiconductor industry currently uses different types of semiconductor-based imagers, such as charge coupled devices (CCDs), complementary metal oxide semiconductor (CMOS) devices, photodiode arrays, charge injection devices and hybrid focal plane arrays, among others.
Solid-state image sensors, also known as imagers, were developed in the late 1960s and early 1970s primarily for television image acquisition, transmission, and display. An imager absorbs incident radiation of a particular wavelength (such as optical photons, x-rays, or the like) and generates an electrical signal corresponding to the absorbed radiation. There are a number of different types of semiconductor-based imagers, including CCDs, photodiode arrays, charge injection devices (CIDs), hybrid focal plane arrays, and CMOS imagers. Current applications of solid-state imagers include cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detector systems, image stabilization systems and other image based systems.
These imagers typically consist of an array of pixel cells containing photosensors, where each pixel cell produces a signal corresponding to the intensity of light impinging on that element when an image is focused on the array. These signals may then be used, for example, to display a corresponding image on a monitor or otherwise used to provide information about the optical image. The photosensors are typically photogates, phototransistors, photoconductors or photodiodes, where the conductivity of the photosensor or the charge stored in a diffusion region corresponds to the intensity of light impinging on the photosensor. The magnitude of the signal produced by each pixel cell, therefore, is proportional to the amount of light impinging on the photosensor.
Active pixel sensor (APS) imaging devices are described, for example, in U.S. Pat. No. 5,471,515, which is herein incorporated by reference. These imaging devices include an array of pixel cells, arranged in rows and columns, that convert light energy into electric signals. Each pixel includes a photodetector and one or more active transistors. The transistors typically provide amplification, read-out control and reset control, in addition to producing the electric signal output from the cell.
While CCD technology has a widespread use, CMOS imagers are being increasingly used as low cost imaging devices. A fully compatible CMOS sensor technology enabling a higher level of integration of an image array with associated processing circuits would be beneficial to many digital imager applications.
A CMOS imager circuit includes a focal plane array of pixel cells, each one of the cells including a photoconversion device, for example, a photogate, photoconductor, phototransistor, or a photodiode for accumulating photo-generated charge in a portion of the substrate. A readout circuit is connected to each pixel cell and includes at least an output transistor, which receives photogenerated charges from a doped diffusion region and produces an output signal which is periodically read out through a pixel access transistor. The imager may optionally include a transistor for transferring charge from the photoconversion device to the diffusion region or the diffusion region may be directly connected to or be part of the photoconversion device. A transistor is also typically provided for resetting the diffusion region to a predetermined charge level before it receives the photoconverted charges.
In a CMOS imager, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to a floating diffusion region accompanied by charge amplification; (4) resetting the floating diffusion region to a known state; (5) selection of a pixel cell for readout; and (6) output and amplification of a signal representing the pixel cell charge. Photo-charge may be amplified when it moves from the initial charge accumulation region to the floating diffusion region. The charge at the floating diffusion region is typically converted to a pixel output voltage by a source follower output transistor.
Each pixel cell receives light focused through one or more micro-lenses. Micro-lenses on a CMOS imager help increase optical efficiency and reduce cross talk between pixel cells. A reduction of the size of the pixel cells allows for a greater number of pixel cells to be arranged in a specific pixel cell array, thereby increasing the resolution of the array. In one process for forming micro-lenses, the radius of each micro-lens is correlated to the size of the pixel cell. Thus, as the pixel cells decrease in size, the radius of each micro-lens also decreases.
The micro-lenses refract incident radiation to the photosensor region, thereby increasing the amount of light reaching the photosensor. Other uses of micro-lens arrays include intensifying illuminating light on the pixel cells of a non-luminescent display device such as a liquid crystal display device to increase the brightness of the display, display associated with a camera, forming an image to be printed in a liquid crystal or light emitting diode printer, and as focusing means for coupling a luminescent device or a receptive device to an optical fiber.
One problem with image devices are the creation of artifacts. Penetration of infrared (IR) radiation to the substrate may create artifacts in the image sensors. Modern image devices usually use so called “dark pixels” that are shielded from incident light and serve as reference pixels for black level calibration, dark current subtraction, and row wise noise correction. IR radiation in the spectral range of from about 800 nm to about 1150 nm, with a very small absorption in the substrate, can penetrate through the entire substrate, be reflected from the backside of the wafer (as well as from the reflectance surface under the die), and hit the “dark pixels.” The absorption of reflected IR radiation by dark pixels can create image artifacts in the modern image devices, despite the fact that the absorption itself is very small in this spectral range. Because of the small number of dark pixels used to calculate the reference signal, usually the dark reference signal is calculated from averaging of 32 or 64 dark pixels, a small change in signals from dark pixels can create large image artifacts. Reflected IR radiation or IR radiation penetrating through the backside of the wafer can create general image artifacts as well.
Reference is made to FIG. 1, which schematically illustrates a solid state imager 10 of the prior art and illustrates the problem of accumulation of reflected IR radiation in the dark pixels. The imager 10 includes active pixels 12 and dark pixels 11. IR radiation 101 is focused through a mirco-lens 13 to the active pixel 12. Some of the radiation 101 passes through the substrate 18 and is reflected from the backside of the substrate 18 as reflected radiation 103 and accumulates in the dark pixel 11. As noted above, the absorption of reflected IR radiation by dark pixels 11 can create image artifacts in the modem image devices, despite the fact that the absorption itself is very small in this spectral range.
The problem of reflected IR radiation in the 800 nm to 1150 nm range is increased for thinner background wafers due to the smaller total optical path of IR radiation hitting dark pixels. Thus, more IR radiation with shorter wavelengths can reach “dark pixels” due to reflection from backside of the substrate when the substrate is thinner.
The present invention discloses a substrate that substantially reduces image artifacts from IR radiation penetrating into the substrates in an image sensor. The present invention improves image quality at extreme light conditions, significantly reduces image artifacts due to interaction of reflected IR light with dark pixels, and allows the use of imaging devices with thinner substrates.